Neural recording and stimulation through micro-machined devices has been a long standing endeavour of researchers and micro-system engineers in order to improve the understanding of neural activity and to achieve purposeful modulation of neural activity, such as neuro-stimulation for the treatment of Parkinson disease, epilepsy and chronic pain. Such devices are, however, particularly challenging from both a fabrication and a signal processing point of view. One of the key components of these devices is the electrode element which interfaces the neural cells to such electronic micro-device. High charge storage capacity and low impedance are desirable properties of the electrode element for good stimulation of electro-genic cells and good data acquisition relating to the cell state, both in in vivo applications, e.g. in neural implants, and in in vitro applications, e.g. in multi-electrode array cell assays.
Platinum, titanium nitride (TiN), and metal oxides like iridium oxide (IrOx) are known in the art as potential electrode materials. TiN is an especially promising material, as it advantageously combines biocompatibility and complementary metal-oxide-semiconductor (CMOS) compatibility. Furthermore, TiN offers good thermal and chemical stability. However, in order to meet the requirements for providing good stimulation and recording, highly porous TiN layers may be required to provide a sufficiently large contact area. Unfortunately, the pore resistance limits the benefits of the increased electrochemical interface area, particularly for narrow and deep pores. Moreover, TiN is less suitable for prolonged stimulation purposes as it forms a stable and insulating surface oxide.
Carbon nano-materials have also been considered as an alternative electrode material because of their good electrochemical stability as well as their high surface to volume ratio. Carbon nanotubes (CNTs) may be the most extensively studied materials of this class, and have proven to be an advantageous material choice for neural recording and stimulation devices. However, while CNTs provide good cell adhesion, their applicability may be hampered by poor adhesion between the CNTs and the substrate. Furthermore, capillary interactions between CNTs in a wet environment may reduce the effective surface area. Another disadvantage of CNTs may be that the surface density of a CNT array can lie in the range of 1% to 5% due to free space between the tubes.
For example, Scott Miserendino et al. disclose, in “Electrochemical characterization of parylene-embedded carbon nanotube nanoelectrode assays”, published in Nanotechnology 17(4), a parylene-embedded carbon nano-tube nano-electrode array in an electrochemical detector. Such array can advantageously be fabricated in a process which is compatible with standard micro-electromechanical system (MEMS) processing and which does not require additional chemical/mechanical polishing.
However, other carbon allotropes may also provide good surface to volume ratios and electrochemical stability, while being more robust than CNTs, and may thus be potentially better suited for neural recording than typical thin-film electrode materials.